Promotion of expression of foreign genes is the most required technique for applying the genetic engineering techniques to plants. This technique includes utilization of a DNA fragment having an activity to promote gene expression. Known DNA fragments which promote expression of foreign genes include the intron of maize alcohol dehydrogenase (Callis et al. Gene & Development 1, 1183-1200 (1987)), and the first intron of rice phospholipase D (hereinafter also referred to as “PLD”) (International Publication WO96/30510). Further, the influences on the activity to promote gene expression, by deleting a part of an internal region of an intron or by inserting the same intron into a site within the intron, have been reported (Mascarenhas et al. Plant Mol. Biol. 15, 913-920 (1990), Clancy et al. Plant Sci.98, 151-161 (1994)).
However, so far, the number of DNA fragments which may be used for this purpose is limited, and in most cases, their gene expression-promoting effects are insufficient. Therefore, a DNA fragment having higher activity has been demanded. Further, although it has been tried to increase the expression-promoting activity by modifying the intron sequences, the region having the activity to promote expression in an intron has not been reported, and a case wherein the promotion activity of an original intron-originated DNA fragment is doubled is not known.